The present invention relates to a catalyst having a porous support, buffer layer and interfacial layer; methods of making the catalyst; and catalytic processes utilizing the catalyst.
Hydrogen and hydrocarbon conversion reactions including such as steam reforming, water-gas shift reactions, methanol synthesis and catalytic combustion are well known. These reactions are usually carried out at temperatures between 150 and 1000xc2x0 C. Currently these reactions are industrially run using catalyst pellets which consist of an active catalytic metal or metal oxide deposited on high surface area ceramic pellets.
Foam or monolith catalysts are known that have three layers (1) porous support, (2) interfacial layer, and (3) catalyst metal as described in [1]. In making these catalysts, the interfacial layer has been deposited by various methods including solution impregnation techniques. The catalyst layer may be deposited by solution impregnation techniques. The interfacial layer has greater surface area than the porous support whereas the porous support has greater mechanical strength than the interfacial layer.
The porous support may be a metal or ceramic foam. Metal foams are highly thermally conductive and easy to machine. The sponge-like mechanical properties allow convenient sealing in a reaction chamber via mechanical contact. The closely matched thermal expansion between the metal foam and the housing reaction chamber minimizes cracking of the porous support and minimizes gas channeling around the porous support at higher reaction temperatures. Pestryakov et al prepared metal foam supported transition metal oxide catalysts with [1] and without [2] an intermediate gamma-alumina layer for the oxidation of n-butane. Kosak [3] examined several approaches to disperse precious metals on various metal foams where the surface was pre-etched with HCl solution, and reported that electroless deposition provides the best adhesion of precious metals to the foam supports. Podyacheva et al. [4] also synthesized foam metal supported LaCoO3 perovskite catalyst with a porous alumina intermediate for methane oxidation. Despite all of the potential advantages with metal foam supported catalysts, metal foam has low corrosion resistance and its nonporous and smooth web surfaces have provided poor adhesion to ceramic materials, and these materials are prone to spalling of interfacial layers after thermal cycling because of the mismatch in thermal expansion.
In order to increase corrosion resistance, methods such as diffusion alloying with Al, Cr, and Si have been used to fabricate ferritic steels, which are typically used for the manufacturing of high temperature furnace elements (about 1200xc2x0 C.) [5]. When the aluminum containing ferritic steels are appropriately heat-treated, aluminum migrates to the alloy surface and forms a strongly adhering oxide film which is resistant to oxygen diffusion. Such ferritic steel foils have been used to fabricate metal monoliths with  greater than 10 ppi (pores per inch) open cells [6]. However, the search for the similar alloy foams with pores suitable for catalytic applications ( less than 20 ppi, 80 ppi preferred) has been fruitless. This has been attributed to both the immature methods for making the finer Al-ferritic steel foams and the lack of the alloy precursors for making the foams.
Hence, there is a need in the art of supported catalysts for a porous support of a foam that is resistant to corrosion or oxidation and resists cracking of the interfacial layer.
References
1. A. N. Pestryakov, A. A. Fyodorov, V. A. Shurov, M. S. Gaisinovich, and I. V. Fyodorova, React. Kinet.Catal.Lett., 53 [2] 347-352 (1994).
2. A. N. Pestryakov, A. A. Fyodorov, M. S. Gaisinovich, V. P. Shurov, I. V. Fyodorova, and T. A. Gubaykulina, React.Kinet.Catal. Lett., 54 [1] 167-172 (1995).
3. J. R. Kosak. A Novel Fixed Bed Catalyst for the Direct Combination of H2 and O2 to H2O2, M. G.Scaros and M. L.Prunier, Eds., Catalysis of Organic Reactions, Marcel Dekker, Inc. (1995), p115-124.
4. O. Y. Podyacheva, A. A. Ketov, Z. R. Ismagilov, V. A. Ushakov, A.Bos and H. J.Veringa, React.Kinet.Catal.Lett., 60 [2] 243-250 (1997).
5. A. N. Leonov, O. L. Smorygo, and V. K. Sheleg, React.Kinet.Catal.Lett., 60 [2] 259-267 (1997).
6. M. V. Twigg and D. E. Webster. Metal and Coated-Metal Catalysts, A Cybulski and J. A. Moulijn, Eds., Structured Catalysts and Reactors, Marcel Dekker, Inc. (1998), p59-90.
The present invention includes a catalyst that has at least three layers, (1) porous support, (2) buffer layer, (3) interfacial layer, and optionally (4) a catalytically active layer. In some embodiments, the buffer layer, which is disposed between the porous support and the interfacial layer, contains at least two compositionally different sublayers. The buffer layer typically provides a transition of thermal expansion coefficient from the porous support to the interfacial layer thereby reducing thermal expansion stress as the catalyst is heated to and cooled from high operating temperatures. The buffer layer also reduces corrosion and oxidation of the porous support, and minimizes side reactions catalyzed by the surface of the porous support.
The invention also provides a catalyst having a porous support, a buffer layer disposed between the porous support and an interfacial layer; and wherein the catalyst possesses oxidation resistance such that, if it is heated at 580xc2x0 C. in air for 2500 minutes the catalyst increases in weight by less than 5%. Alternatively, the catalyst may also be characterized by its resistance to flaking during thermal cycling.
The invention further provides a process of converting at least one reactant to at least one product in which the reactant is passed through a reaction chamber containing the catalyst.
The method of the present invention for making the multi-layer catalyst (at least three layers) has the steps of (1) selecting a porous support, (2) depositing a buffer layer on the porous support, (3) depositing an interfacial layer thereon, and optionally (4) depositing a catalytically active layer onto or integral with the interfacial layer; wherein the buffer layer is disposed between the porous support and the interfacial layer. Better results can be obtained where the buffer layer is vapor deposited. The catalytically active layer can be deposited after or during the deposition of the interfacial layer.
Advantages of the present invention, that include a porous support with a buffer layer and an interfacial layer, may include: better match of thermal expansion coefficients and better stability to temperature changes, reduction of side reactions such as coking, desired metal-oxide interactions, strong bonding to a high-surface-area interfacial layer, and enhanced protection of the underlying porous support.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.